Serial cycle threshold to assess the infectious potential of SARS-CoV-2: A systematic review.

We sought to assess predictive factors for SARS-CoV-2 infectiousness using a meta-analytic approach. We searched LitCovid, medRxiv, Google Scholar, and the WHO COVID-19 database until June 30 2025, including studies which cultured SARS-CoV-2, relating them to clinico-epidemiologic and laboratory variables and RT-PCR cycle threshold (Ct) values. Using linear mixed-effects regression models, we tested for independent associations with Ct values with 95%CIs and adjusted P-values in a multivariable model. We used a modified QUADAS criteria to assess risk-of-bias. We included 50 studies, with 39 in quantitative synthesis. The percentage of culture-positive specimens decreased with increasing Ct values (subgroup test difference Q = 96.71;P < 0.001) and time since the first PCR test (Q = 26.95;P = 0.0026). Presence of symptoms (Q = 20.1;P < 0.01), gene platform used (Q = 14.89;P = 0.002), being a cancer patient (Q = 24.9;P < 0.0001), and vaccination status (Q = 8.80;P = 0.012) were associated with increased culture-positivity, whereas a rising Ct (adjusted Ct change -6.58[95%CI] -5.30, -7.86;P < 0.001) was strongly associated with culture-negativity. Analysing 186 immunocompetent patients with 1,393 Ct values, 2 consecutive Cts ≥ 30 or a rising Ct value on serial testing demonstrated a sensitivity of 87.5% and specificity of 96.3% using culture positivity as the outcome. Serial Ct monitoring, integrated with clinico-epidemiologic data is a valuable tool for assessing infectiousness, providing objective criteria for discontinuing isolation and guiding clinical decisions.